This invention relates generally to an electrostatographic printing machine, and more particularly concerns an improved development system for use therein.
In the process of electrostatographic printing, an electrostatic latent charge pattern is recorded and reproduced in viewable form. The field of electrostatography includes electrophotographic and electrographic printing. Electrophotographic printing employs electromagnetic radiation to form the electrostatic latent image or charge pattern. Electrographic printing forms the latent image without the aid of electromagnetic radiation. In both of the foregoing processes, the latent image must be rendered viewable, i.e. developed with particles. Hereinafter electrophotographic printing will be discussed as an illustrative embodiment of the foregoing.
An electrophotographic printing machine employs a photoconductive member which is charged to a substantially uniform level to sensitize its surface. The charged portion of the photoconductive member is exposed to a light image of the original document being reproduced. As a consequence of the exposure, the charge is selectively dissipated in the irradiated areas in accordance with the light intensity reaching the surface. This records an electrostatic latent image on the photoconductive surface. The latent image recorded on the photoconductive surface is developed by bringing a developer mix into contact therewith.
Typically, a developer mix comprises dyed or colored thermoplastic powders, known in the art as toner particles, mixed with coarser carrier granules, i.e. ferromagnetic granules. The toner particles and carrier granules are selected such that the toner particles acquire the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. As the developer mix is brought into contact with the latent image, the greater attractive force thereof causes the toner particles to transfer from the carrier granules thereto. This concept was originally disclosed by Carlson in U.S. Pat. No. 2,297,691 being further amplified and described by many related patents in the art. Heretofore, development systems have employed rotary impellers, fur brushes, bucket conveyors and magnetic brush systems to develop the electrostatic latent image. A magnetic brush system achieves a high degree of uniform toner deposition. Usually, a magnetic brush system includes a developer roll having a directional flux field for advancing the magnetizable developer mix into contact with the latent image.
Multi-color electrostatographic printing records a series of different electrostatic latent images on the photoconductive surface, each latent image corresponding to a particular color in the original document. In such a system, there is a need to develop each of the latent images, i.e. single color latent images, with toner particles complementary in color to the color of the filtered light image transmitted to the photoconductive surface.
In multi-color development systems, a plurality of developer units are required. The developer units each furnish the appropriately colored toner particles to the latent image. If all of the developer units are continuously in operation the resultant toner powder image will contain a mixture of colors from each of the developer units, i.e. the colors will be inter-mingled with one another. This will result in the toner powder image being mis-colored with the resultant multi-color copy lacking the appropriate color balance. Therefore, it is apparent that only one development system may be in operative communication with the latent image at a given time. One method of solving this problem is to articulate the developer units so that only one developer unit at a given time is in operative communication with the latent image. This technique is more fully described in U.S. Pat. No. 3,854,449 issued to Davidson in 1974. Other references, i.e. U.S. Pat. Nos. 3,570,453, 3,575,139 and 3,641,969 teach the articulation of a blade into contact with the developer roll to prevent developer mix from being moved into the development zone. However, no prior art appears to teach the concept of a fixed mounting multi-color development system wherein the developer roll applies toner particles to the latent image and acts as a clean-up magnet to remove residual or unwanted carrier granules therefrom.
Accordingly, it is the primary object of the present invention to improve the development system of an electrostatographic printing machine.